Phenolic resin dispersions are commercially important materials that are used in an array of diverse products. Examples include coatings, such as anti-corrosive baking enamels for metals and coatings used as food contact surfaces; adhesives; contact adhesives or tack-building agents in many acrylic, vinyl acrylic, vinyl and rubber latexes; binders for organic and inorganic materials; laminates; moldings; and other uses.
Aqueous phenolic resin dispersions made with gums or cellulosic materials as the dispersing agent were found to exhibit poor storage stability. Moreover, the coatings resulting from such dispersions had unacceptable chemical resistance due to the hydrophilic nature of the dispersing agent. Surfactants, such as phosphate esters, have also been tried with limited success, as surfactant-containing dispersions provide unacceptably poor film properties caused by surface migration of the surfactant.
Stable dispersions of phenolic resin polymers have been achieved using a protective colloid material, such as polyvinyl alcohol (PVA), as a dispersant in U.S. Pat. No. 4,400,229. Likewise, novolac resin dispersions are stabilized with PVA in U.S. Pat. No. 4,788,236. Protective colloids can also be used in systems relying on an organic coupling solvent for the phenolic resin, as described in U.S. Pat. Nos. 4,124,554 and 5,200,455. More recently, a stable aqueous dispersion of a hydrophilic phenolic resin has been achieved in U.S. Pat. Nos. 5,552,186 and 5,548,015 by the incorporation of highly hydrophobic etherified bisphenol-A into an aqueous solution of a phenolic resin and a PVA protective colloid.
In addition to their stability in dispersions, however, another major concern associated with phenolic resin polymers in general is the level of their free formaldehyde content. Typical amounts of free formaldehyde in such resins cause formaldehyde to be released into the environment during processing, storage of the treated substrate, and use. Such formaldehyde release is undesirable, especially in an enclosed space. In particular, formaldehyde may be inhaled by workers and may come in contact with the eyes, the mouth, and other parts of the body. Therefore, ways to reduce the level of free formaldehyde in resins and resin dispersions in order to correspondingly lower the formaldehyde emissions resulting from the further processing of these resins and/or from their end products are beneficial.
In addressing the problem of reducing free formaldehyde in certain polymer dispersions, U.S. Pat. No. 6,753,383 teaches the addition of a thioamine compound.
However, rather than dispersions of phenolic polymers (i.e., compositions comprising a phenolic polymer that is dispersed within a separate continuous phase), significantly greater attention has been directed to formaldehyde reduction in phenolic polymer solutions (i.e., single phase compositions). For example, U.S. Pat. No. 5,684,118 teaches the scavenging of formaldehyde in formaldehyde-based resins using a low mole ratio, storage stable, melamine-urea-formaldehyde (MUF) resin additive. Otherwise, the MUF resin, which itself has a low level of formaldehyde emissions, may be used alone to make composite boards, as taught in U.S. Pat. No. 5,681,917.
U.S. Pat. No. 5,889,137 teaches the use of a phenol aralkylation polymer, derived from a phenolic monomer, at least one styrene derivative, and an aryl diolefin, to significantly reduce formaldehyde emissions.
Both U.S. Pat. Nos. 5,864,003 and 6,730,770 teach the modification of phenolic resin resoles by reaction with one or more aldehyde scavengers to lower the amount of free aldehyde in such resins.
Finally, U.S. Pat. No. 6,706,845 teaches the use of a relatively large quantity of catalyst compared to typical amounts in laminating resins, in combination with one or more formaldehyde scavengers during later portions of the A-stage of the resin cook, to yield a phenol formaldehyde resole having low free formaldehyde and low formaldehyde emissions during B- and C-stage resin processing and curing.
In contrast to solutions, however, dispersions are susceptible to losing their homogeneity over time due to phase separation by settling, coalescence, agglomeration, etc. of the dispersed phase. Therefore, storage stability, or the long-term ability of a dispersion to maintain homogeneous, well-mixed dispersed and continuous phases, is a major consideration affecting the practical utility and thus ultimately the salability of a dispersion. As is well known in the art generally (and particularly with respect to the above-noted attempts to find additives that yield stable dispersions), the addition of any component to a dispersion can have an unpredictable effect on both the dispersion stability and the ultimate properties of the materials made from the dispersion. Indeed, many factors, acting both independently and interactively, are known to affect dispersion stability, including zeta potential (i.e., inter-particle electrostatic attraction or repulsion), particle size, pH, and solution conductivity. Therefore, the addition of any agent to potentially effect a reduction in free formaldehyde content can also impact any or all of these factors, possibly resulting in destabilization of the dispersion.
In view of the above, there is a need in the art to identify phenolic resin dispersion additives that not only reduce free formaldehyde to more environmentally friendly thresholds, but also provide stable dispersions at economically attractive levels of solids content, viscosity, etc. Likewise, such additives should ideally not affect the desirable characteristics, including reactivity, moisture resistance, strength, etc. of end products of the phenolic resin dispersions.